Numerous investigations have been reported of attempts to produce colloidal base metals, particularly iron, cobalt and nickel. In this respect or in this context, the term "colloidal" refers to exceptionally small metal particles in which the size of the particles is less than about 1,000 nanometers (nm). Although the formation of these base metals from a solution of the corresponding metal salt with a strong reducing agent is known in the art, the results reported show either large particles (greater than 1,000 nm in size), or an impure product, and additionally, and most significantly, the metal particles produced are generally irregular in shape, or of a spherical, or isometric morphology. For example, Sapieszko et al. in Corrosion, Vol 36, pp 522-530, 1980, reported the hydrothermal (or hydrometallurgical) reduction of chelated nickel and cobalt by the reduction of the corresponding M.sup.2+ ion from basic solutions. The process was conducted in an autoclave at high pressure and at 250.degree. C., using a hydroxide concentration of 1.2 molar, a chelating agent such as ethylenediamine tetraacetic acid, and a reducing agent such as hydrazine or triethanol amine. A process of this type conducted under high pressures and elevated temperatures is potentially hazardous. Moreover, the cobalt and nickel particles produced by the process appear to be very irregular in shape, or of a spherical, or isometric morphology, and mostly of a size larger than 1000 nanometers (nm).
There have been attempts to synthesize particulate base metals from basic solutions at moderate temperatures (i.e., 100.degree. C. or less) using hydrazine as the reducing agent, as disclosed in U.S. Pat. No. 4,214,893 to Harada et al. However, the process requires the use of a palladium catalyst (Pd.sup.+2), a protein, and a magnetic field, which, as reported by Matijevic et al. in Plating and Surface Finishing, Vol. 62, pp. 958-965, 1975, invariably yields colloidal palladium coated by the base metal. Thus, the actual products of the palladium catalyzed reaction are cobalt coated palladium particles. In addition, the particles are spherical in shape.
Because aqueous iron, cobalt, and nickel cations are difficult to reduce, strong reducing agents must be employed. There are several reports discussing the use of sodium borohydride as the reducing agent, but the resulting products are spherical in shape and contain substantial amounts or are nearly all of the metal boride.
The prior art shows the electroless deposition of a metal ion, particularly nickel, to the corresponding metal film using basic solutions of hydrazine as the reducing agent. The final product, however, is a film and not a particle, and the use of a catalyst such as palladium is required.
Ultrasound has been used to produce colloidal metal, for example, by atomizing a molten metal, as reproted by Pohlman, et al., Ultrasonics, Vol 12, pp 11 et seq., 1974. The sonication of iron carbonyl and of cobalt carbonyl nitrosyl yields powders of amorphous metal, as reported by Suslick, et al, in Nature, Vol 353, pp 414-416, 1991, and by Suslick, MRS Bulletin, Vol 20, pp 29-34, 1995. The resulting metal particles are relatively large in size (greater than 1,000 nm), and irregular in shape.
No reference is known for the facile synthesis of unsupported base metal particles of iron, cobalt, and nickel of relatively small size from a solution or suspension of a salt of the corresponding metal at atmospheric pressure. Further, the prior art does not show the formation of such particles exhibiting a morphology that is essentially nonspherical and anisometric.
This invention has therefore as its purpose to provide for the facile synthesis of unsupported base metal particles of iron, cobalt, and nickel exhibiting anisometric morphology.
It is another object of the invention to provide for the formation of unsupported base metal particles of iron, cobalt, and nickel exhibiting anisometric morphology from a solution or suspension of a salt of the corresponding metal.
It is yet another object of the invention to provide for the formation of agglomerates from these base metal particles.